A typical long-range optical transmission system includes a pair of unidirectional optical fibers that support optical signals traveling in opposite directions. An optical signal is attenuated over long distances. Therefore, the optical transmission line will typically include repeaters that restore the signal power lost due to fiber attenuation and are spaced along the transmission line at some appropriate distance from one another. The repeaters include optical amplifiers. The repeaters also include an optical isolator that limits the propagation of the optical signal to a single direction.
In long-range optical transmission links it is important to monitor the health of the system. For example, monitoring can detect faults or breaks in the fiber optic cable, localized increases in attenuation due to sharp bends in the cable, or the degradation of an optical component. Amplifier performance should also be monitored. For long haul undersea cables there are two basic approaches to in-service monitoring: monitoring that is performed by the repeaters, with the results being sent to the shore station via a telemetry channel, and shore-based monitoring in which a special signal is sent down the transmission path or line and which is received and analyzed for performance data. Coherent optical time domain reflectometry (COTDR) is one shore-based technique used to remotely detect faults in optical transmission systems. In COTDR, an optical pulse is launched into an optical fiber and backscattered signals returning to the launch end are monitored. The time between pulse launch and receipt of a backscattered signal is proportional to the distance along the fiber to the source of the backscattering, thus allowing the fault to be located. In the event that there are discontinuities such as faults or splices in the fiber, the amount of backscattering generally changes and such change is detected in the monitored signals. Backscattering and reflection also occur from discrete elements such as couplers, which create a unique signature. The link's health or performance is determined by comparing the monitored COTDR trace with a reference record trace. New peaks and other changes in the monitored signal level are indicative of changes in the fiber path, normally indicating a fault.
One complication that occurs when COTDR is used in a multi-span transmission line in which the individual spans are concatenated by repeaters is that the optical isolators located downstream from each repeater prevent the backscattered and/or reflected signal from being returned along the same fiber on which the optical pulse is initially launched. To overcome this problem each repeater includes a bidirectional coupler connecting that repeater to a similar coupler in the opposite-going fiber, thus providing an optical path for the backscattered light so that it can be returned to the COTDR unit. In most DWDM links employing such a return path there may also be a filter immediately following the coupler so that only the COTDR signal is coupled onto the return path, thus avoiding interference that would occur if the signals from one fiber were coupled onto the return path fiber). Thus, signals generated by the backscattering and reflection of a COTDR pulse launched on one fiber are coupled onto the opposite-going fiber to be returned to the COTDR unit for analysis.
In addition to monitoring the health of the undersea cable, it is important to monitor the health of the optical amplifiers located in the repeaters. Typically, this monitoring is performed by the repeaters themselves, with the results being sent to the shore station via a telemetry channel. If instead COTDR could be used in-service to identify and locate optical amplifier faults, the telemetry channel could be eliminated.
Accordingly, it would be desirable to provide a method and apparatus for determining from COTDR traces the identity and location of one or more optical amplifier faults that arise along an optical transmission path.